NAND flash memory has been proposed as a main storage element to replace hard disk drives, which have been used for a long time in the Personal Computer (PC) systems and servers. NAND flash memory uses “tunnel injection” for writing and “tunnel release” for erasing. Such writing and erasing makes use of a quantum tunneling effect, also called Fowler-Nordheim tunnel injection, wherein charge carriers are injected into an electric conductor through a thin layer of an electric insulator (a gate oxide).
Recent expansion in the use of NAND flash memory in the semiconductor memory system may be, at least in part, attributed to a relatively low power consumption feature that makes NAND flash memory particularly suitable for mobile products.
NAND flash memory is arranged as strings of flash memory cells. Associated with each NAND memory cell string is a bit line. Running across the NAND memory cell strings are word lines. Accordingly, through selection of, i.e., application of an appropriate voltage on, a particular bit line and a particular word line, a particular flash memory cell may be selected for writing.
When programming a flash memory cell, a program voltage is applied to a control gate of the flash memory cell and the bit line, associated with the NAND memory cell string that includes the flash memory cell, is grounded. Electrons from a p-well are injected into a floating gate of the flash memory cell. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the flash memory cell is raised. To apply the program voltage to the control gate of the flash memory cell being programmed, that program voltage is applied on the appropriate word line. The word line is also connected to a control gate of one flash memory cell in each of the other NAND memory cell strings that utilize the same word line. A problem arises when it is desired to program one flash memory cell on a word line without programming the other flash memory cells connected to the same word line. Because the program voltage is applied to the control gate of all flash memory cells connected to a word line, an unselected flash memory cell (a flash memory cell that is not to be programmed) on the word line, especially a flash memory cell adjacent to the flash memory cell selected for programming, may become inadvertently programmed. The unintentional programming of the unselected flash memory cell on the selected word line is referred to as “program disturb”.
Several techniques can be employed to prevent program disturb. In one method, known as “self boosting”, the bit lines that are not selected are electrically isolated and a pass voltage (e.g., 10 volts) is applied to the word lines that are not selected during programming. The unselected word lines couple to the unselected bit lines, causing a voltage (e.g., eight volts) to exist in the channel of the unselected bit lines, thereby tending to reduce program disturb. Self boosting causes a voltage boost to exist in the channel. The voltage boost tends to lower the voltage across the tunnel oxide and reduce program disturb.
Recent improvements in process technology have allowed for smaller transistors and a reduction in the main supply voltage (VDD) level. Such a reduction in VDD level acts to prevent transistor destruction due to the high electrical stress for the thin gate oxide tunneling operation.
However, the reduction in VDD level has reduced the usefulness of the self-boosting programming method described above wherein unselected bit lines are electrically isolated. To prevent program disturb in the channel of the flash memory cell connected to the word line to which a program high voltage (Vpgm) has been applied, an associated program inhibit bit line voltage (at least VDD) should be held as high as possible.